The present invention relates to a magnetic recording and playback device of a helical scan system in which both a video signal and an audio signal are recorded on or played back from a magnetic tape utilizing rotary heads, and more particularly to a tracking apparatus for a magnetic recording and playback device in which an audio signal is recorded on a deep layer of a magnetic tape and a video signal is recorded on a shallow layer of the magnetic tape.
In order to have a good quality played back audio signal, even in a long-play mode in which a magnetic tape is run slower than in a normal mode, the technique in which recording and playback of an audio signal is performed with a rotary head is known. The technique is, for example, described in Japanese Patent Application Laid-Open No. 38907/84, in which a video head for recording and playback of video signals and an audio head for recording and playback of audio signals are mounted on a rotating cylinder, and after an audio signal is recorded on a magnetic tape, a video signal is recorded on the same track. In this case an audio signal is recorded on a deep layer of the magnetic tape and a video signal is recorded on a shallow layer.
FIG. 9 shows the track pattern on a magnetic tape 1 formed as described above wherein 88 is a control track, 89 is an audio track and 90 is a video track. In this figure, an audio head precedes to forms an audio track 89 on the magnetic tape and thereafter a video head form a video track 90 over the audio track 89. In this case, an audio signal is recorded on a deep layer of the magnetic tape 1 and a video signal is recorded on a shallow layer. A control track 88 is formed by a control head. In a playback mode, an audio head scans the audio track 89 for the playback of an audio signal and a video head scans the video track 90 for the playback of a video signal. At this time, a control signal is played back from the control track 88 with the control head and the tracking control of an audio head and a video head is performed with the control signal.
An example of an automatic tracking system is shown in Japanese Patent Application Laid-Open No. 41114/79 wherein it is intended to obtain good tracking even in the cases of occurrence of expansion and contraction of a magnetic tape, variations in height of control heads among magnetic recording and playback devices, or variation in the tape tension. A control signal is formed from the phase difference between a pulse expressing the rotary phase of a rotating cylinder and a playback control signal, wherein the phase of a playback control signal is automatically adjusted corresponding to the level of a played back video signal, and tracking control is performed with the phase-controlled control signal and the pulse expressing the rotary phase of the rotating cylinder. It is, therefore, made possible that a video head can obtain good tracking even in the cases where there are different variations as mentioned above.
When a video signal and an audio signal are recorded as shown in FIG. 9, the relative position between a video track 90 and an audio track 89 is set to be a fixed value for the interchangeability among magnetic recording and playback devices. But because of variations in head width, in level differences of heads or in dispositions of heads, it is difficult to make the relative positions between the tracks 89 and 90 of all magnetic recording and playback devices be the same and some differences among the devices cannot be avoided. In particular, the difference in the relative positions becomes large in a long-play recording mode.
When a magnetic tape which is recorded with a different magnetic recording and playback device is to be played back, if the relative position between an audio track 89 and a video track 90 has a fixed value, it is theoretically possible to put an audio head correctly on an audio track and to put a video head correctly on a video track with the above-mentioned prior art. Actually, however, there is variation in the construction or differences in head positions among different magnetic recording and playback devices, so that it is essentially impossible to ensure that a video head and an audio head are placed in correct tracking conditions simultaneously. In other words, if a video head is placed in a good tracking condition, a tracking error is caused in an audio head, and if an audio head is placed in a good tracking condition, a tracking error is caused in a video head.